1. Field of the Invention
The present invention relates to a semiconductor sensor such as semiconductor acceleration sensors, semiconductor flow sensors and semiconductor micro-valves, and a method of making the same.
2. Description of the Related Art
In semiconductor sensors such as semiconductor acceleration sensors, semiconductor flow sensors and semiconductor micro-valves, a silicon semiconductor substrate is selectively etched to provide a thin portion or a penetrating aperture therein. As a technique of etching such a silicon semiconductor substrate, there has been employed an electrolytic etching method using caustic potash, which utilizes a difference in electrolytic potentials due to the conductivity types of the semiconductor substrate. FIG. 1 shows etching characteristics obtained when N-type and P-type silicon semiconductor substrates having a (100) plane are electrolytically etched using the caustic potash. In FIG. 1, the ordinate indicates the electric current, and the abscissa the voltage, respectively. As is apparent from FIG. 1, in the case of the N-type semiconductor, the etching proceeds until the voltage reaches about 2 V. When the voltage exceeds 2 V, the etching is stopped. On the other hand, in the case of the P-type semiconductor, the etching proceeds until the voltage rises to about 4 V, but the etching stops when the voltage exceeds 4 V. Thus, if the voltage is set at 3 V, the N-type semiconductor is not etched, while the P-type semiconductor is etched.
By means of this anisotropic etching, the thin portion or penetrating aperture is formed in the silicon semiconductor substrate, thus providing a semiconductor sensor. FIG. 2 shows a structure of a conventional semiconductor acceleration sensor, which comprises a P-type silicon semiconductor substrate 11, an N-type semiconductor region 13 serving as a thin portion (diaphragm) and formed in a major surface of the P-type semiconductor substrate 11, a P-type semiconductor region 14 serving as a resistor and formed in the N-type semiconductor region 13, an electrode wiring layer 15 formed on the P-type semiconductor region 14 through an insulating layer 12, a funnel-shaped cavity 16 made from the bottom surface of the semiconductor substrate 11 to form the thin portion, and a funnel-shaped aperture 17 penetrating in the thickness direction of the substrate 11 so as to surround both side surfaces of the thin portion.
The funnel-shaped aperture 17 is formed by etching the bottom surface of the semiconductor substrate 11 so as to penetrate the substrate 11. The angle between the bottom surface of the semiconductor substrate 11 and the wall defining the aperture 17 is about 60.degree.. Thus, as is shown in FIG. 3, when the thickness of the semiconductor substrate 11 is given by h, the diameter of the funnel-shaped aperture 17 becomes 1.2 h. As a result, it is not possible to reduce the size of the semiconductor sensor.